Seiren

Seiren is an "Underwater Combat Angeloid, Type Eta" (水中戦闘用エンジェロイドタイプη（イータ）) built by the Sky Master who appeared in chapter 45 of the manga. Her appearance in the manga was brief, as she was impaled in the back by one of Chaos' wings before being devoured and absorbed by her just after she was ordered by the Sky Master to kill Tomoki and Ikaros. Being absorbed granted Chaos water resistance. Her appearance is based on the sirens of Greek mythlology